1. Field of the Invention
The present invention relates to a device for bending plastic printing plates in preparation for mounting the plate on an offset press.
2. Description of the Prior Art
The rapid growth in the use of offset lithography is due, in large part, to the speed and low cost of making the image carrier or plate, which is used for printing the multiple copies on the press. In recent years, the evolution of photodirect image plates has accelerated this growth. These plates are actually the film used in lithographic cameras to photograph the original material or copy. Unexposed areas on the plate are dissolved in water or a solvent, leaving the exposed portion as an ink-receptive image area or stencil.
The developed plate is mounted to a plate cylinder on the press. In use, the cylinder rotates, engaging the plate against a dampening roller to wet the plate, so that the non-printing areas on the plate will repel the ink. Next, the plate contacts an inking roller which transfers ink to the receptive image areas. Subsequently, the image is transferred from the plate to a rubber blanket cylinder, which finally transfers the image to the paper stock.
Consequently, the alignment of the image on the plate relative to the plate cylinder is highly important to insure that the final printed image is transferred to a correct position on the paper. Typically, the plate is fixed to the cylinder by bending an edge segment of the plate and installing the bent segment into a peripheral slot on the cylinder. The line of the bend is related to the leading edge of the image which is the first portion of the plate to contact the blanket cylinder. The opposite, trailing end of the plate is wrapped around the plate cylinder and also installed in the slot adjacent the leading edge.
As a result, the lead edge bend of the plate must be accurately positioned relative to the image on the plate. Metal printing plates are bent by common sheet metal bending tools having positioning means engageable with the leading edge of the plate. However, plastic printing plates are very resilient, have a significant memory, and cannot be permanently bent using ordinary bending apparatus for metal plates. In contrast to metal plates, plastic plates must be forcefully crimped by reversely folding the edge segment about an arc and into contact with an adjacent area of the sheet. Subsequently, a die must be used to permanently crease the sheet adjacent the fold, in order to overcome a substantial part of the memory of the sheet along the line of fold.
Previously, plastic plate benders have utilized a flat base having a positioning means, or stop engageable with the trailing edge of the sheet. A swingable die was then moved to engage the leading edge, and would continue to move to reversely fold and crimp the leading edge segment to achieve the bend. Unfortunately, while the stop used on these bending devices engaged the sheet at its trailing edge, the plate image had been previously aligned relative to the leading edge. Thus, if the trailing edge of the sheet was not precisely parallel with the leading edge, the final printed image would be askew on the paper stock.
Furthermore, such prior art benders necessarily restricted the use of plates of varying lengths, since the stops must engage the trailing edge of the sheet. Benders having adjustable stops required a time-consuming changeover, and the use of such adjustable stops often compounded alignment errors. Alternatively, the user could purchase a bender for each plate size. Obviously, there has long been a desired need in the art for a plastic plate bender which utilizes the leading edge of the sheet to locate the position of the bend.